(1) Field of the Invention
The present invention relates to an electrically insulating, one-component type developer for electrophotography. More particularly, the present invention relates to an electrically insulating, magnetic one-component type developer, in which cohesion of developer particles is prevented and the flowability of developer particles is improved, and which can provide a developed image having an enhanced image density and a reduced fog density.
(2) Description of the Prior Art
As the developer capable of developing an electrostatic latent image without using a particular carrier, there is known a so-called one-component magnetic developer comprising a powder of a magnetic material contained in developer particles.
In this one-component type magnetic developer, there arise problems of cohesion of developer particles and reduction of the flowability of developer particles, which are not encountered in case of a two-component type developer. In the two-component type developer, toner particles having a relatively small particle size are electrostatically attracted to a magnetic carrier having a relatively large particle size and in this state, both the toner and carrier are supplied to a developing sleeve. In the developing sleeve, the stirring effect is given by revolution of the magnetic carrier. Therefore, cohesion of toner particles or reduction of the flowability of toner particles hardly become serious problems in the two-component type developer.
In the one-component type magnetic developer, however, since there is not contained a magnetic toner exerting the above function, there is a prominent tendency of developer particles to cohere to one another, and if this cohesion once takes place, the cohesion state is left without being broken, readily causing such troubles as reduction of the flowability of the developer and reduction of the image density. Especially in case of an electrically insulating one-component type magnetic developer comprising a magnetic material, optionally with a charge control agent, dispersed in an electrically insulating binder medium, since developer particles are readily charged by friction, electrostatic cohesion is readily caused to occur, and reduction of the flowability or image quality is a serious problem.